1. Field of the Invention
The invention relates to a fuel cell assembly with at least one proton exchange membrane (PEM) fuel cell for generating electrical energy from the reactant gases hydrogen and oxygen, comprising at least one membrane/electrode unit which has a membrane coated with platinum electrodes and has, respectively positioned on each side thereof, a porous gas diffusion layer, or which has a membrane and, respectively positioned on each side thereof, a porous gas diffusion layer that is coated with a platinum electrode, and further comprises bipolar plates which lie against the gas diffusion layers and through which, during operation, a coolant flows. The invention further relates to a method for operating a fuel cell assembly of this type.
2. Description of the Related Art
In proton exchange membrane (PEM) fuel cells, in an electrochemical process, electric current, heat and water are formed from hydrogen and oxygen. The fundamental construction is configured such that gas chambers and cooling chambers in “bipolar plates” adjoin a membrane/electrode unit (or membrane electrode assembly (MEA)). The membrane/electrode unit contains, in particular, a proton-conducting membrane that is coated on both sides with platinum electrodes (catalyst layer). These, in turn, are covered by a gas-permeable, electrode-conducting gas diffusion layer. Alternatively, the gas diffusion layer can also be coated with the platinum electrode (catalyst layer) on a side facing toward the membrane. The gas diffusion layer also has the task of removing the product water on the cathode side from the production zone on the border layer with the platinum electrode and the membrane. For this purpose, the gas diffusion layer, which is typically made of carbon fiber material (carbon paper, carbon fiber fabric or nonwoven fabric), is made hydrophobic on the surface, i.e., on the carbon fibers or in the hollow spaces.
Heat produced during fuel cell operation is usually removed from the bipolar plate by a coolant flow, in particular a cooling water flow. As a result, a thermal gradient forms in the bipolar plate from the coolant entry to the coolant exit, i.e., a higher temperature prevails in the region of the coolant exit than at the coolant entry. The reactant (oxygen and hydrogen) exit is also often situated in the region of the coolant exit. On the oxygen side, a large quantity of product water arises that must be completely removed from the gas diffusion layer. On the hydrogen side, in this region, the hydrogen can be effectively converted due to the very good flow. With this, in the region of the conversion of the hydrogen which is already heated by the coolant flow, additional waste heat arises, which can lead to a further temperature rise.
In an ideal case, there is an evenly rising temperature gradient between the coolant entry and exit with little or no temperature rises at the corners or edges of the bipolar plate. In reality, however, at sites critical to flow dynamics (e.g., dead zones, corners) significant temperature increases (for example, 10 to 20 Kelvin as compared with the coolant exit) can arise. This effect can be further strengthened in these critical regions if the coolant water flow of the bipolar plate is significantly reduced, for example, due to faults (blocking of cooling channels, or unintended reduction of the coolant water pump output). In an extreme case, this can lead via different mechanisms (for example, low humidity—formation of hydrogen peroxide and consequent chemical attack on the membrane, mechanical loading by means of humid/dry cycles) to mechanical weakening of the membrane or thickness reduction and even hole formation, which leads to the failure of the cell and thus of the overall fuel cell stack.
Conventionally, the problem is typically handled by attempting, with a suitable optimization of the flow geometry of the bipolar plate, to prevent such hot points.
U.S. Pat. No. 8,617,760 B2 describes another solution. According to this document, the proton-conducting membrane is deactivated in the critical regions via the incorporation of metal ions.
U.S. 2009/0162734 A1 discloses a fuel cell assembly with a PEM fuel cell, where over the entire edge region of the membrane/electrode unit, i.e., the entire region around the outer periphery of the membrane/electrode unit, access by at least one of the reactant gases to the membrane is blocked by a resin layer. The platinum electrode has a smaller area than the gas diffusion layer, where over the entire edge region of the membrane/electrode unit, the gas diffusion layer protrudes beyond the platinum electrode. With this protrusion, in conjunction with a seal extending round the entire edge region of the membrane/electrode unit and the resin layer extending round the entire edge region of the membrane/electrode unit, the contact between the gas diffusion layer and the membrane/electrode unit and their sealing can be improved.
U.S. 2006/127738 A1 discloses a fuel cell construction in which, over the entire edge region of the membrane/electrode unit, an adhesive is arranged between the membrane and the gas diffusion layers. As the adhesive, a substance, such as acrylic or thermoplastic elastomers, can be used. The presence of the adhesive reduces the stretch loading at the edges of the membrane that are not supported by electrodes. The adhesive acts as a sealant and thus prevents a chemical degradation of the membrane. The gas diffusion layers are porous. As a result, the adhesive can penetrate into the pores of the gas diffusion layers.